15.3.1 Basic Principles
Microgravimetry is an emerging branch of science developed on the basis of gravimetry. Therefore, the basic theories and concepts of microgravity potential fields are basically the same as those of gravity, and have their uniqueness. However, in terms of particularity, the nature and characteristics of "micro" are highlighted. It is an exploration method based on the earth's gravitational field and studies the changes in density of different lithologies to solve some special geological problems.
Microgravity measurement is different from conventional gravity measurement. It is a gravity measurement that can achieve micro-G level accuracy. In order to ensure the analysis and analysis results with micro-G level accuracy, the key lies in the methods and technologies of field survey operations, which have many different requirements, special measures and regulations from conventional survey surveys, and are much more complicated than conventional gravity measurements. In terms of geology and other natural conditions, the influence of factors such as terrain, landform, near-instrument objects, temperature, pressure, vibration, solid tide, etc.; in terms of observation operation technology, factors such as the placement of instruments and chassis, adjustment operations, and measurement point elevation are all required Special considerations; recording methods also require special provisions. For data obtained from microgravity observations, in addition to the same items as the correction of conventional gravity observation data, in order to ensure that the quality requirements of microgravity-level observation data are met, corrections for the effects of near objects and buildings within a certain range are also required. Correction of effects.
As we all know, all objects on the surface of the earth and nearby space have weight, which is the result of the gravity of the objects. Point P0 is any point on the earth. There is a particle (object) with mass m0 at P0. See Figure 15-3. It is subject to the gravitational force F(M,mo) produced by the earth with mass M on particle m0; at the same time, particle m0 It is also affected by the inertial centrifugal force C (m0) generated as the earth rotates. The direction of the inertial centrifugal force points outward perpendicular to the earth's rotation axis. The resultant force G(M, m0) of the vector combination of gravity and inertial centrifugal force is gravity.
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Figure 15-3 Earth’s Gravity Field
The direction of gravity points slightly differently in different locations. Because the direction of gravity G(M,m0) roughly points to the center of the earth.
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Particle Poo is not only attracted by earth materials, but also by other celestial materials such as the sun and moon. Under the influence of the gravitational pull of the sun and moon, the moving earth's gravity will also undergo periodic slight changes over time.
The space where the effect of gravity exists is called a gravity field.
In order to facilitate comparative research on the distribution of matter inside the earth, the gravity experienced by unit mass is used as the research standard, which is called gravity field strength or gravitational acceleration. The measurement of gravitational acceleration is simply called gravity measurement. Gravity measurement can be divided into absolute measurement and relative measurement. Absolute gravity measurement measures the full value of gravity, which is called absolute gravity value; relative gravity measurement measures the gravity difference of each point relative to a certain reference point. Relative gravity measurement is the main form of modern measurement.
The acceleration due to gravity on the Earth's surface varies from location to location. The main content of microgravity exploration is to study the characteristics of underground geological structures based on the measured gravity changes on the ground, and to explore mineral deposits, underground man-made structures and some remains of human activities. Due to the force deformation of rocks, differences in underground caves, etc. will produce changes in the microgravity field. By studying this change, we can achieve the purpose of detecting geological disasters, such as landslides, collapses, ground subsidence, etc.
Generally, the main reasons for changes in surface gravity acceleration are:
(1) The actual shape of the earth is relatively complex, with the north pole slightly protruding, the south pole retracting, and the equatorial radius slightly larger than the pole radius. A large, pear-shaped oblate sphere, and the ground is undulating;
(2) The earth rotates around a certain axis of rotation;
(3) The interior of the earth, especially the crust The density distribution of the lithosphere and its nearby materials is uneven. This is the result of many complex geological processes in the history of the earth. Therefore, this unevenness is closely related to geological structures and mineral distribution;
< p>(4) The existence of relics and man-made structures formed close to the surface by human historical activities has caused slight changes in density distribution in local areas.15.3.2 Observation methods
The methods of measuring gravity can be divided into dynamic methods and static methods. The dynamic method is to observe the movement of an object under the action of gravity, and the directly measured quantities are time and distance; the static method is to observe the balance of an object, and the directly measured quantities are the linear displacement and angular displacement of the object due to changes in gravity.
Figure 15-4 Simple working principle of the gravimeter
Using the static method to measure relative gravity is the main method of gravity exploration, and the instrument used is a gravimeter. According to different measurement methods, gravity measurement can be divided into gravity measurement and gravity vertical gradient measurement. Gravity measurement refers to directly measuring the gravity acceleration (absolute value or relative value) of the measuring point; gravity vertical gradient measurement refers to measuring the rate of change of the earth's gravity along the vertical direction.
Figure 15-4 is a simple working principle diagram of the gravimeter. The original length of the spring is S0, its upper end is fixed on the bracket, and a load of mass M is suspended from the lower end.
Under the action of gravity gG, the length of the spring extends from So to SG, so there is: K is the elastic coefficient of the spring. If it is moved to another point A, the length of the spring under the action of gravity gA at that point is SA, then
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S0 remains unchanged In this case, the gravity difference between points A and G can be determined by the following formula
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where: is the constant of the instrument, which is equivalent to a change in the length of the spring. The change value of gravity per unit time is called the grid value of the gravimeter; △S is the length change of the spring length at points A and G. Therefore, when the grid value C is known, if the change of the spring length at two points can be accurately measured, the gravity difference between the two points can be calculated.
When the absolute gravity value on the reference point is known, the absolute gravity value of the observation point can also be obtained through relative gravity measurement, that is:
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15.3.3 Technical requirements
15.3.3.1 Classification and point layout principles of microgravity measurements
In engineering, microgravity measurements can generally be divided into two categories : ① Profile measurement, the profile is generally perpendicular to the set trend of linear underground structures (such as faults, anticlines, synclines and hidden rivers); ② Area measurement, mainly detecting the size, shape and distribution of underground geological bodies. Regardless of section or area measurement, the relative elevation of the gravity measuring point location must be determined using geodetic methods.
The field procedures used for exploration depend on the purpose of the exploration and the requirements for data correction. Measurements in microgravity exploration are carried out relative to reference points in the local area and do not require determination of absolute gravity values. . As for the scale in area measurement, it can be determined according to the needs of the project, ranging from 1:200 to 1:1000.
Principles of point layout for microgravity measurements:
(1) Arrange the detected objects or anomalies in the center of the survey line or survey area;
(2) ) The survey line or survey area should be as close to the geological body related to the detection object as possible;
(3) The direction of the survey line should be as perpendicular to the direction of the detection object as possible, and as close as possible to the known geological The profiles are consistent;
(4) The distance between measuring points should be less than 1/2 to 1/3 of the width of the credible anomaly, ensuring that at least four measuring points can reflect the above anomalies;
(5) The distance between survey lines shall not be greater than 1/2 to 1/3 of the projected length of the geological body on the ground.
15.3.3.2 Geodesy work in microgravity measurement
(1) Tasks of geodesy work
The main tasks of geodesy are: ①According to microgravity The requirements for gravity survey design are to lay out survey lines or survey nets (area measurement) in the work area, determine the coordinates of the survey points in order to draw maps and make normal gravity (latitude) corrections; ② Determine the elevation of the survey points in order to conduct spatial (height) , Intermediate layer correction (of course, the density of rock and soil is also required to be measured); ③ In areas with undulating terrain, topographic measurements of corresponding scales are required to facilitate land reform.
(2) Methods and requirements for geodesic work
Methods and requirements for geodesic work are: ① Use a theodolite or range finder to measure the coordinates of the gravity point, which can be attached to National grid (point) or independent coordinates; ② Use a level or distance meter to measure the elevation of a gravity point, and the accuracy can meet the requirements of level IV, and the elevation should be affiliated with the national elevation system; ③ When doing topographic survey, if The elevation accuracy near the gravity point (0~4m) is about 1cm, the accuracy at 4~10m is about 2cm, the accuracy at 10~100m is about 5cm, and it can be slightly worse above 100m. The final calculated ground correction accuracy may reach 3×10 -8m·s-2; ④ When conducting underground microgravity measurements, in addition to measuring the point position and elevation according to the above requirements, it is also necessary to measure the position and elevation of each section of the tunnel in order to make corrections to the tunnel; ⑤ When making microgravity measurements or gradient measurements close to buildings such as walls, stone pillars, and instrument piers, their relative positions, shapes, sizes, etc. need to be measured in order to make corrections for near-instrument objects and buildings.
15.3.3.3 Requirements and record contents of microgravity measurement field records
(1) Recording items of microgravity measurement record book
Microgravity measurement The items recorded in this record should include the following content according to their characteristics: ① Optical shift sensitivity; ② Reading line; ③ Transportation method; ④ Instrument name and number; ⑤ Readings at both ends of the longitudinal bubble; ⑥ Readings at both ends of the horizontal bubble; ⑦ Gravity reading Time and reading; ⑧The distance between the ground (measuring point pile) and the bottom edge of the instrument; ⑨Air pressure, air temperature and internal temperature of the instrument; ⑩Description of external interference, including wind and vibration; (1 point) position description; (12 measurement) point surroundings Topography and landform description.
(2) Recording items in the near-instrument object measurement record book
Since the measurement of near-instrument object and the topography and landform measurement in the survey area can be carried out simultaneously, the near-instrument object record book It can also be used for on-site topographic and geomorphological measurements in nearby areas.
The record should record the following contents: ① A plan sketch of the work area, which includes the plans and numbers of all objects to be measured, and has their orientation; ② A sketch and number of each object to be measured, and the number should be consistent with the number of the plan sketch Consistent and with orientation; ③ If the sketch of the object to be measured is divided into several regular geometric bodies, a detailed drawing of each segmented body must be drawn, and the number of the segmented body shall be consistent with the number of the sketch, and shall be consistent with the number on the recording paper The numbers are consistent, and each geometric object in the detailed drawing is marked with a position mark and a density mark for use in measurement, and it must have an orientation.
15.3.4 Compilation of Microgravity Observation Data
Since microgravity measurements require very high precision, that is, micro-G level precision, various types of measurements must be made during observations. Many corrections, corrections and processing are required before processing calculations, analysis and interpretation.
15.3.4.1 Processing and correction of observation data
The observation value gi of a measuring point can be expressed by the following formula:
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In the formula: gi is the converted gravity value at the measuring point; f(zi) is the conversion of the reading value zi into a gravity value based on the grid value table and calibration value (linear, quadratic term) Conversion (grid value) function; Cm is the magnetic field coefficient, which can be calibrated from the laboratory; mg is the magnetic field strength. If the instrument is strictly oriented north at each measuring point and avoids strong magnetic field interference, this item can be ignored; CT is the temperature coefficient, which can be calibrated from the laboratory; mT is the temperature difference, which can generally be ignored; δ is the tidal factor, which varies from region to region and is generally taken as 1.16; GT is the theoretical value of solid tide at the time of observation; P is the period Number of errors; A. is the periodic error amplitude, is the periodic error angular frequency, Tn is the period, and φn is its phase. These can be calibrated in the baseline field, but the current calibration is generally not accurate enough, so they are not often used for correction; αp is the air pressure coefficient, △P is the difference between the actual measured air pressure and the standard air pressure P(H). The last term is the change in the balance position of the instrument pendulum (gravity reading) caused by changes in air pressure. This can be done in a decompression chamber and the correction coefficient CpP can be obtained. If the air pressure change mp is known, this can be obtained item correction. However, according to experiments with some gravity instruments, this effect is very small and can be ignored in microgravity measurements.
15.3.4.2 Normal gravity correction, height (space) correction and intermediate layer correction
(1) Normal gravity correction: For microgravity measurements, a reference latitude can usually be specified for the base point , and then use the following formula to calculate the latitude correction of all other measuring points:
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In the formula: △gZL is in μGal; △L is the distance from the base point (or reference point) north-south distance, in m; φ is the reference latitude; if the measuring point to be corrected is south of the base point, use a positive sign, if it is in the north, use a negative sign.
(2) Height (space) correction: Since microgravity measurement is relative to an arbitrary reference elevation (the elevation of the base point, or the elevation of the geoid, or the elevation of the mean sea level), and only The measuring point elevation needs to be relative to the reference elevation, so the height (spatial) correction formula is:
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In the formula: △gZFA is in μGal; △h is the height difference between the measuring point that needs to be corrected and the reference elevation, in m; the positive sign is used for measuring points that are higher than the reference elevation, and the negative sign is used for measuring points that are lower than the reference elevation.
(3) Intermediate layer correction (i.e. Bouguer correction): For the intermediate layer Bouguer correction, a reference elevation should be selected, preferably the same reference elevation as the height (spatial) correction, and each If the distance between a measuring point and the reference elevation is approximated by the material of an infinite horizontal plate, the Bouguer correction formula is:
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In the formula: ΔgZ Bouguer Calibration is in μGal; ρ is the density of the plate (g/cm3); Δh is the elevation difference between the corrected measuring point and the reference elevation, in m; when the measuring point is higher than the reference elevation, the negative sign is taken, and vice versa Take the positive sign.
15.3.4.3 Terrain Correction
Topography correction is extremely important for microgravity measurements and is the main factor affecting the calculation of gravity anomalies. There are three main calculation methods for terrain correction.
(1) Surface integration method: The basic principle of the surface integration method is to calculate the volume integral of gravity terrain correction and convert it into a surface integral calculation formula for the terrain surface and all horizontal planes of the terrain correction point according to Gauss' theorem. Triangular surfaces are used to fit terrain relief, and the integral of each triangular unit is numerically multiplied using Gaussian formula. The advantages of this method are high accuracy, fast calculation speed and great flexibility. It can be used for far zone, middle zone and near zone correction.
(2) FFT ground correction calculation: The FFT ground correction calculation method is the fast Fourier transform terrain correction calculation method. It is characterized by its simple formula and easy to be quickly implemented on a computer.
(3) Zoning calculation method: The zoning calculation method divides the terrain correction range into near area, middle area and far area. The triangular prism model with inclined top surface is used in the near area, and the square prism formula is used in the middle and far areas.
15.3.4.4 Correction of the influence of near-instrument objects on microgravity measurements
(1) Correction of the gravity effect of the observation instrument pier: The observation instrument pier is the object closest to the instrument. For gravity measurements The influence cannot be ignored. Generally, cylinders, truncated cones, and square cylinders are used as geometric models for correction. As for the walls or cliffs around the instrument, they can be combined with models such as square cylinders (cubes, rectangular cylinders), and the gravity effects are calculated and corrected based on their measured density.
(2) Correction of the influence of buildings: Microgravity measurements are often carried out in building groups, even at the feet of buildings and inside buildings. The influence of huge building mass can also be called the influence of "near instrument mass". Since the general shape of buildings is mostly regular geometry, when considering its impact, the building can be decomposed into a combination of several cuboids (including rhombus), cylinders, spheres, and prisms. If the building is divided into fine enough parts and the respective gravity values, gravity vertical gradient values, etc. are calculated using the effect theoretical formulas of the corresponding regular bodies (rectangular, cylinder, sphere, etc.), the building can be calculated more accurately. The overall gravity effect, gravity field distribution and corresponding correction values.
15.3.5 Data processing of microgravity measurements
The main purposes of microgravity data processing are:
(1) Eliminate the problems caused by gravity measurement and gravity measurement As a result, some errors are introduced during various corrections, or the interference of certain near-surface small density inhomogeneous bodies that are irrelevant to the purpose of exploration is eliminated;
(2) Superimposed anomalies caused by multiple geological factors , classify anomalies related to gravity exploration targets;
(3) Convert potential fields according to the needs of gravity exploration problems.
15.3.5.1 Curve Smoothing
Curve smoothing is used to eliminate field gravity measurement observation errors and errors caused by various corrections to the measurement results.
(1) Freehand smoothing method: Experienced technicians directly smooth abnormal curves according to their changing patterns. During freehand smoothing, it should be noted that the deviation of the gravity anomaly value of each corresponding point before and after smoothing should not exceed the mean square error of the measured anomaly, and the areas formed by the abnormal curves before and after smoothing should be equal as much as possible, and the center of gravity should remain unchanged.
(2) Multiple averaging method: Use the average gravity anomaly of two adjacent points as the anomaly value of the midpoint of the two points, and then smooth the curve freehand until the desired smoothness is finally reached.
(3) Smoothing formulas for profile anomalies: including linear smoothing formulas and quadratic curve smoothing formulas.
Linear smoothing formula:
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The smoothing value of a certain point is taken as an odd number of points on the profile with that point as the center. Arithmetic mean. From m=1, 2, 3... we can get 3, 5, 7... point smoothing formulas respectively.
Quadratic curve smoothing formula: including five-point and seven-point smoothing formulas.
The five-point smoothing formula is:
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The seven-point smoothing formula is: (4) Smoothing formula for plane anomalies: linear Smoothing formula (see above).
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Five-point smoothing formula:
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Nine-point smoothing Formula:
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15.3.5.2 Division of regional anomalies and local anomalies
Regional anomalies are generally caused by relatively deep burial, Or caused by residual mass with a large distribution range; local anomalies are generally caused by relatively shallow or small geological bodies. Before conducting geological interpretation, especially quantitative interpretation, it is necessary to process the superimposed anomalies and divide them into regional anomalies and local anomalies. The commonly used methods are:
(1) Graphical method: Graphical method is divided into two types: parallel straight line method and smooth curve method. The parallel straight line method is suitable for areas where regional gravity anomalies change linearly along the horizontal direction; smoothing method The curve method is suitable for regional gravity anomaly contours which cannot be represented by parallel straight lines but can only be represented by curves.
(2) Numerical calculation methods: including deviation method, circle method, network method, etc.
(3) Polynomial fitting method and trend analysis method.
15.3.5.3 Potential field conversion
Potential field conversion is mainly to facilitate the processing of inverse problems. The main contents include:
(1) From the observation plane The gravity observation values ??on the plane are converted into the second-order and third-order partial derivatives (Vxz, Vzz, Vzz2) of the gravity anomaly on the same plane, that is, the derivative conversion of the gravity anomaly.
(2) From the gravity observation values ??on the observation plane, △g, Vxz, Vzz, Vzz2, etc. at any point other than the anomaly source are converted into analytical extensions of the gravity anomaly.
15.3.5.4 Microgravity measurement data inversion method
The inversion of microgravity measurement data is the basis for the quantitative interpretation of microgravity anomalies. Before inversion, it is necessary to carefully analyze the superimposed anomalies and try to extract the gravity anomalies related to the exploration targets, so that it is possible to make a quantitative explanation of the geological bodies that caused the anomalies.
(1) Analytical method: We know that △g, Vxz, Vzz and Vzz2 of a geological body are functions of its occurrence elements, remaining mass and observation point coordinates.
On the contrary, if the occurrence elements or remaining mass of geological bodies are expressed as functions of gravity anomalies (or their derivatives) and observation point coordinates, then when the △g (or its derivatives of each order) produced by these geological bodies is known , then the occurrence factors and remaining quality parameters of the geological body can be obtained based on this functional relationship. The calculation methods include △g abnormal curve solution and Vxz, Vzz, Vzz2 curve solution.
(2) Tangent method: Use the tangent of the abnormal curve characteristic point to find the approximate burial depth of the top (or center) of the object using a graphical method.
(3) Selection method: Based on the basic characteristics of the profile anomaly curve of the measured gravity anomaly or the distribution and change of the gravity anomaly contour on the gravity anomaly plane map, combined with the geology and other geophysical data of the work area, the selection method is selected. Develop a model of the geological body that causes this gravity anomaly, and use the problem-solving method to calculate the theoretical anomaly of the model body, and then compare the theoretical anomaly with the measured anomaly. When the two are within the allowed error range, then the The model of a given geological body is the solution sought.
(4) Direct method: directly use the distribution of gravity anomalies on the profile curve or plan view, and solve certain parameters of the abnormal body through integral operations, such as the remaining mass of the three-dimensional body, the coordinates of the center of mass or the two-dimensional The cross-sectional area and center of mass coordinates of the body, etc.
(5) Inversion of density interface: Determining the fluctuation of underground density interface based on measured gravity anomalies is very important for studying geological structures. For this work to achieve good results, the following conditions must be met: ① The gravity anomaly used for inversion calculation is caused by the fluctuation of the density interface; ② The density distribution of the material layers above and below the interface is relatively uniform, and their densities are known Poor; ③ The interface depth of at least one or several points in the work area is known. Methods for solving the density interface include: linear formula solution method, second-order approximation formula solution method, compressed mass surface method, etc.
(6) Shallow stress field inversion: The calculation formula for calculating the shallow stress field of the earth's crust is derived based on the elastic mechanics equilibrium equation, and the measured gravity data on the earth's surface is used to invert the shallow stress field. , to explore the mechanical mechanisms and stability trends of some geological bodies.
15.3.6 Geological explanation of microgravity anomalies
The geological explanation of microgravity anomalies can be divided into qualitative explanation and quantitative explanation. Qualitative explanation is to judge the geological causes of gravity anomalies based on the basic characteristics of gravity anomalies and known geological and other geophysical data. Quantitative explanation is to perform quantitative calculations on some meaningful anomalies and find out certain occurrences of geological bodies when conditions are met.
Before explaining gravity anomalies, the equivalent sources of gravity anomalies and the resulting multiple solutions to the inverse problem of gravity exploration must be carefully considered. Therefore, when interpreting data, it is necessary to obtain as much information as possible to narrow the scope of the solution.
(1) Make full use of the known geological conditions of the work area, such as types of strata and rocks, structural occurrences, etc., so that the solution to the inverse problem conforms to objective reality as much as possible;
(2) Rock density data is not only the basis for arranging gravity exploration work, but also an important parameter for solving the inverse problem of gravity exploration. It should be carefully collected, analyzed and utilized. If necessary, specimens can be collected for direct measurement or indirectly through surface gravity data and well measurement data. Determination;
(3) Make full use of drilling data to collect the accurate thickness of various formations and the physical properties of various rocks in order to obtain important information needed to explain anomalies;
(4) Various geophysical data can supplement and provide evidence for the explanation of gravity anomalies and should be fully utilized.
15.3.7 Expression form of results
The main forms of results of microgravity measurement include: gravity anomaly plane contour map and gravity anomaly profile curve map; various partial derivative planes, etc. Value lines and section curves; analytical continuation plane contour maps and sections; various inference and interpretation figures, etc.
15.3.8 Outlook
Microgravity measurement is an emerging exploration method. Although its field measurement and data processing are relatively complex, it has the characteristics of not being restricted by terrain and not subject to various It has the characteristics of high sensitivity to electromagnetic influence and abnormal body reflection, and can play a more active role in geological disaster exploration, such as ground subsidence, landslides, debris flows, avalanches, ground fissures, reservoir banks, ground subsidence and other geological surveys. Good application prospects.
15.3.9 Instruments and Equipment
The instruments and equipment for microgravity exploration are shown in Table 15-4.
Table 15-4 List of microgravity measuring instruments